The present invention relates generally to methods for authenticating currency and other certificates, and specifically to methods of verification based on watermarks.
Watermarks are commonly used as a security feature on paper currency, as well as other official certificates. Their advantage in this regard stems from the fact that true watermarks can only be formed during the manufacture of the paper, requiring large, expensive facilities that are not available to most counterfeiters. The watermark comprises areas of high- and low-density paper, created during the manufacturing process. The resulting density differences create an image that is easily visible to the naked eye under back illumination.
Counterfeiters typically attempt to simulate these density differences by means such as printing on the currency bill in a color indistinguishable from the background. Good-quality counterfeit bills produced in this manner may be difficult for the untrained observer to spot. Visual inspection is complicated by the fact that on many types of bills, the watermark is covered by printing on one or both sides of the bill. The watermark may also be obscured (intentionally or not) by dirt and stray markings on the bill. There is therefore a need for automated watermark authentication devices that overcome the limitations of visual inspection.
A variety of methods of currency verification based on automated watermark inspection are known in the art. For example, U.S. Pat. No. 5,854,673, whose disclosure is incorporated herein by reference, describes a method for authenticating watermarked paper based on transiently heating the paper. A thermal imager captures an image of the paper, and the change in the thermal image over time is observed in order to determine whether or not the watermark is authentic. As another example, U.S. Pat. No. 4,296,326, whose disclosure is incorporated herein by reference, describes an apparatus and method for detecting a genuine watermark using ultraviolet radiation. The fluorescence characteristics of a currency note are observed while the note is subjected to the radiation.
U.S. Pat. No. 6,104,036, whose disclosure is incorporated herein by reference, describes an apparatus and method for detecting both a watermark and a security thread in a currency note. (The security thread, commonly used in U.S. notes, indicates the denomination of the note.) Optical sensing circuits positioned on both sides of a currency note measure both transmissive and reflective characteristics. A difference signal between the transmissive and reflective light signals is compared to a series of known difference signals to determine the authenticity of the note.
Similarly, U.S. Pat. No. 5,923,413, whose disclosure is likewise incorporated herein by reference, describes a bank note denominator and validator that operates by comparing transmitted and reflected light signals from a currency note, using sensors positioned on either side of the note. Multiple emitters illuminate the note at different wavelengths in sequence. The measured values of transmission and reflection at the different wavelengths are compared to templates of stored values for known note types. The template matching results are used to determine the denomination of the note and to identify counterfeits.
Other methods of watermark-based currency note authentication are described in Japanese patent publications JP 0113269, JP 1009589, JP 2148382, JP 3191495, JP 7272041 and JP 8287313, whose disclosures are incorporated herein by reference.
Other optical methods for currency authentication are also known in the art. For example, U.S. Pat. No. 5,367,577, whose disclosure is incorporated herein by references, describes a method for testing genuineness of paper bills based on detecting reflected and scattered light from the bills in different narrow wavelength bands. The differences in measured intensity are indicative of differences in the printing process used to make counterfeit notes relative to that used for genuine notes. U.S. Pat. Nos. 3,496,370 and 3,679,314, whose disclosures are also incorporated herein by reference, likewise describe methods for testing bills based on detecting light of different colors that is reflected and/or transmitted by the bills.
It is an object of some aspects of the present invention to provide devices and methods for testing the authenticity of watermarked certificates, and particularly currency bills.
It is a further object of some aspects of the present invention to provide devices for currency authentication that are simple and low in cost while providing reliable detection of counterfeits.
In preferred embodiments of the present invention, an optical bill validator captures an image of a currency bill under test, preferably using a single light source and detector to sense and process light transmitted through the bill. Based on the type and denomination of the bill (which are preferably determined automatically by analyzing the image), the validator chooses a template corresponding to the watermark that should appear on the bill. The validator correlates the watermark area of the bill under test with the template, in order to register the image of the watermark on the bill with the template and to calculate a correlation value indicating the quality of the match between the bill and the template. The validator then determines the range of intensity values in the registered watermark image, and calculates a scaling factor relating this intensity range to that of the template.
To determine the authenticity of the bill, the validator maps the pair of correlation and scaling factor values to a corresponding vector in a multi-dimensional decision space. Optionally, other factors, such as measurements of the color of the bill, may be used as coordinates of the vector, as well. Pixels in the image of the bill that are substantially brighter or dimmer than the template (typically due to marks or stains on the bill) are excluded from consideration. Based on previous testing, a certain region of the decision space is known to contain the vectors corresponding to most or all valid bills of this type and denomination, while excluding all (or nearly all) known counterfeits. If the vector for the current bill under test falls within this certain region, the validator judges the bill to be authentic. Otherwise, the bill is rejected.
Although the preferred embodiments described herein are directed to currency verification, the principles of the present invention may similarly be applied to testing of other types of watermarked paper. In this manner, for example, it is possible to authenticate other certificates of value, such as checks and gift certificates, as well as to carry out quality control inspection of watermarked paper items.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for verifying authenticity of a certificate under test, wherein an authentic version of the certificate has a known watermark in a predetermined watermark region thereof, the method including:
capturing an image of at least a portion of the watermark region of the certificate under test;
finding a correlation between the image of the watermark region and the known watermark;
responsive to the correlation, registering a portion of the image of the watermark region with the known watermark;
measuring a range of intensity values in the registered portion of the image;
defining a vector in a decision space, the vector having coordinates given by the range of the intensity values and by a value of the correlation between the image of the watermark region and the known watermark; and
determining the certificate under test to be authentic if the vector falls within a predefined area of the decision space.
Preferably, capturing the image includes capturing a transmission image by detecting light transmitted through the certificate under test. Alternatively or additionally, capturing the image further includes capturing a reflection image by detecting further light reflected from the certificate under test.
In a preferred embodiment, capturing the image includes illuminating the certificate using first and second light source respectively located on opposing first and second sides of the certificate, and receiving light both from the first source transmitted through the certificate and from the second source reflected from the certificate using a detector on the second side of the certificate. Optionally, illuminating the certificate includes operating the first and second light sources simultaneously, so that the image captured by the detector includes both the transmitted and the reflected light. As a further option, operating the first and second light sources includes operating one of the sources substantially continuously and the other of the sources intermittently, while the detector is receiving the transmitted and the reflected light.
Typically, the certificate includes a currency bill. Preferably, finding the correlation includes determining a denomination of the bill, and choosing the known watermark with which the correlation is to be found from among a set of such known watermarks, responsive to the denomination. In a preferred embodiment, finding the correlation further includes identifying a manufacturer of the bill, and choosing the known watermark from among the set depending on the manufacturer.
Preferably, finding the correlation includes identifying an interfering image element, not belonging to the watermark, in the image of the watermark region, and computing the correlation while excluding the interfering image element from the correlation. Further preferably, measuring the range of the intensity values includes determining the range of the intensity values in the region exclusive of the interfering image element. Additionally or alternatively, identifying the interfering image element includes finding pixels in the image of the watermark region having intensity values outside a predetermined range that is associated with the known watermark. Most preferably, the method includes counting a number of the pixels whose intensity values are outside the predetermined range, and rejecting the certificate if the number exceeds a predefined threshold. Additionally or alternatively, the method includes counting a number of the pixels whose intensity values are within the predetermined range, wherein defining the vector includes defining a further vector coordinate responsive to the number of the pixels.
Preferably, defining the vector includes defining one of the coordinates to be an intensity scaling factor that maps the range of intensity values in the registered portion of the image to the intensity values in the known watermark.
There is also provided, in accordance with a preferred embodiment of the present invention, apparatus for verifying authenticity of a certificate under test, wherein an authentic version of the certificate has a known watermark in a predetermined watermark region thereof, the apparatus including:
an image sensor, which is adapted to capture an image of at least a portion of the watermark region of the certificate under test and to generate an output signal responsive thereto; and
an image processor, coupled to receive the output signal, and adapted to process the signal so as to find a correlation between the image of the watermark region and the known watermark and, responsive to the correlation, to register a portion of the image of the watermark region with the known watermark, the image processor being further adapted to measure a range of intensity values in the registered portion of the image, to define a vector in a decision space, the vector having coordinates given by the range of the intensity values and by a value of the correlation between the image of the watermark region and the known watermark, and to determine the certificate under test to be authentic if the vector falls within a predefined area of the decision space.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: